In a multi-carrier communication using a plurality of carrier waves, a broad and stable frequency characteristic including a DC component is demanded for a transmitter/receiver. Additionally, the demand for a broad effective band and a narrow sub-carrier interval has been increasing in recent systems in order to implement a high data throughput. To meet such demands, a filter, an RF element, etc., which are included in a signal system, must be implemented at a stable frequency characteristic.
If measures for suppressing frequency distortion in a filter, etc. are taken, power consumption generally increases. Moreover, an element must be manufactured with a semiconductor process using SiGe, etc., of a high unit price in order to suppress 1/f noise in the vicinity of DC, leading to an increase in the cost of the system.
FIG. 1 illustrates a receiver for a multi-carrier communication according to conventional technology.
A carrier wave band reception signal that is received by an antenna 10 passes through a transmission/reception switch 11, a carrier wave bandpass filter 12, and a low noise amplifier 13. Then, the frequency of the reception signal is converted by being multiplied in quadrature demodulators 14-1 and 14-2 by local carrier waves that are generated by adjusting the frequency of the output of a local oscillator 19 with a synthesizer 21 and are implemented as a 0° phase wave and a 90° phase wave with a phase shifter 22. Moreover, the output of the local oscillator 19 is generated as a clock signal that is frequency-adjusted by a PLL (Phase Locked Loop) 20, and given to AD converters 17-1 and 17-2. The clock signal is used as a sampling signal in the AD converters 17-1 and 17-2. As the frequency of the clock signal, a frequency double the band of a baseband signal is used. Thereafter, the signals pass through lowpass filters 15-1 and 15-2, and baseband signals are extracted. Their relations are expressed with the following equations. Alphabets enclosed with parentheses respectively indicate a signal passing through a corresponding portion.                carrier wave band reception signal received by the antenna 10 (signal in a portion of (A))[Mathematical Expression 1]        
      s    ⁡          (      t      )        =            ∑              n        =        0                    N        -        1              ⁢          [                                    a            n                    ⁢          cos          ⁢                      {                          2              ⁢                              π                ⁡                                  (                                                            f                      c                                        +                                          nf                      0                                                        )                                            ⁢              t                        }                          -                              b            n                    ⁢          sin          ⁢                      {                          2              ⁢                              π                ⁡                                  (                                                            f                      c                                        +                                          nf                      0                                                        )                                            ⁢              t                        }                              ]      where:    an=An cos θn: InPhase component    bn=An sin θn: Quadrature Phase component    N: Number of: MultiCarrier            local carrier waves generated by the synthesizer 21[Mathematical Expression 2]I-Branch Local Carrier Wave(B)=cos(2πfct)Q-Branch Local Carrier Wave(C)=cos(2πfct−π/2)=sin(2πfct)        reception signals the frequency of which is converted by the quadrature demodulators 14-1 and 14-2[Mathematical Expression 3](D) s Reception Signal        
                                          S            I                    ⁡                      (            t            )                          =                ⁢                              s            ⁡                          (              t              )                                ⁢                      cos            ⁡                          (                              2                ⁢                π                ⁢                                                                  ⁢                                  f                  c                                ⁢                t                            )                                                              =                ⁢                              ∑                          n              =              0                                      N              -              1                                ⁢                      [                                                            a                  n                                ⁢                cos                ⁢                                  {                                      2                    ⁢                                          π                      ⁡                                              (                                                                              f                            c                                                    +                                                      nf                            0                                                                          )                                                              ⁢                    t                                    }                                ⁢                                  cos                  ⁡                                      (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        c                                            ⁢                      t                                        )                                                              -                                                                      ⁢                              b            n                    ⁢          sin          ⁢                      {                          2              ⁢                              π                ⁡                                  (                                                            f                      c                                        +                                          nf                      0                                                        )                                            ⁢              t                        }                    ⁢                      cos            ⁡                          (                              2                ⁢                π                ⁢                                                                  ⁢                                  f                  c                                ⁢                t                            )                                      ]                                =                ⁢                                            1              2                        ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                              [                                                                            a                      n                                        ⁢                                          cos                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                                                              -                                                            b                      n                                        ⁢                                          sin                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                                                                            ]                                              +                                                ⁢                              1            2                    ⁢                                    ∑                              n                =                0                                            N                -                1                                      ⁢                          [                                                                    a                    n                                    ⁢                                      cos                    ⁡                                          (                                                                        4                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            c                                                    ⁢                          t                                                +                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                                    )                                                                      -                                                      b                    n                                    ⁢                                      sin                    ⁡                                          (                                                                        4                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            c                                                    ⁢                          t                                                +                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                                    )                                                                                  ]                                          (E) Q-Branch Reception Signal
                                          S            Q                    ⁡                      (            t            )                          =                ⁢                              s            ⁡                          (              t              )                                ⁢                      {                          -                              sin                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      c                                        ⁢                    t                                    )                                                      }                                                  =                ⁢                              ∑                          n              =              0                                      N              -              1                                ⁢                      [                                                            a                  n                                ⁢                cos                ⁢                                  {                                      2                    ⁢                                          π                      ⁡                                              (                                                                              f                            c                                                    +                                                      nf                            0                                                                          )                                                              ⁢                    t                                    }                                ⁢                                  {                                      -                                          sin                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            c                                                    ⁢                          t                                                )                                                                              }                                            -                                                                      ⁢                              b            n                    ⁢          sin          ⁢                      {                          2              ⁢                              π                ⁡                                  (                                                            f                      c                                        +                                          nf                      0                                                        )                                            ⁢              t                        }                    ⁢                      {                          -                              sin                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      c                                        ⁢                    t                                    )                                                      }                          ]                                =                ⁢                                            1              2                        ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                              [                                                                            a                      n                                        ⁢                                          sin                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                                                              -                                                            b                      n                                        ⁢                                          cos                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                )                                                                                            ]                                              -                                                ⁢                              1            2                    ⁢                                    ∑                              n                =                0                                            N                -                1                                      ⁢                          [                                                                    a                    n                                    ⁢                                      sin                    ⁡                                          (                                                                        4                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            c                                                    ⁢                          t                                                +                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                                    )                                                                      -                                                      b                    n                                    ⁢                                      cos                    ⁡                                          (                                                                        4                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            c                                                    ⁢                          t                                                +                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                          n                          ⁢                                                                                                          ⁢                                                      f                            0                                                    ⁢                          t                                                                    )                                                                                  ]                                                          baseband signals after passing through the lowpass filters 15-1 and 15-2[mathematical expression 4](F) I-Branch Baseband Reception Signal        
                                                                                                              s                    ~                                    I                                ⁡                                  (                  t                  )                                            =                            ⁢                                                1                  2                                ⁢                                                      ∑                                          n                      =                      0                                                              N                      -                      1                                                        ⁢                                      [                                                                                            a                          n                                                ⁢                                                  cos                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        )                                                                                              -                                                                        b                          n                                                ⁢                                                  sin                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        )                                                                                                                ]                                                                                                                          =                            ⁢                                                1                  2                                ⁢                                                      ∑                                          n                      =                      0                                                              N                      -                      1                                                        ⁢                                      [                                                                  A                        n                                            ⁢                                              cos                        ⁡                                                  (                                                                                    2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        +                                                          θ                              n                                                                                )                                                                                      ]                                                                                                          (                  1          ⁢                      -                    ⁢          1                )            where:                an=Ancosθn         bn=Ansinθn (G) Q-Branch Baseband Reception Signal        
                                                                                                              s                    ~                                    Q                                ⁡                                  (                  t                  )                                            =                            ⁢                                                1                  2                                ⁢                                                      ∑                                          n                      =                      0                                                              N                      -                      1                                                        ⁢                                      [                                                                                            a                          n                                                ⁢                                                  sin                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        )                                                                                              -                                                                        b                          n                                                ⁢                                                  cos                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        )                                                                                                                ]                                                                                                                          =                            ⁢                                                1                  2                                ⁢                                                      ∑                                          n                      =                      0                                                              N                      -                      1                                                        ⁢                                      [                                                                  A                        n                                            ⁢                                              sin                        ⁡                                                  (                                                                                    2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              n                              ⁢                                                                                                                          ⁢                                                              f                                0                                                            ⁢                              t                                                        +                                                          θ                              n                                                                                )                                                                                      ]                                                                                                          (                  1          ⁢                      -                    ⁢          2                )            where                an=Ancosθn         bn=Ansinθn         
The baseband signals converted from the multi-carrier signal pass through variable gain amplifiers 16-1 and 16-2, are sampled by the AD converters 17-1 and 17-2, and converted into information for respective sub-carriers by an FFT unit 18. d(0) to d(N−1):{d(n)=an+jbn} at the right of FIG. 1 represent the reception signals of the respective sub-carriers. N is the number of sub-carriers used, and the similar to the size of the FFT.
FIG. 2 illustrates the spectrum of the multi-carrier communication reception signal received by the antenna 10.
The sub-carriers are allocated at the frequency interval of fo[Hz], centering on the frequency of the carrier wave fc[Hz]. If N sub-carriers are used, the frequency band of the entire multi-carrier signal is Nfo[Hz]. d(0) to d(N−1) of FIG. 2 correspond to those of FIG. 1, and represent the symbols of the respective sub-carriers. d(0) and d(N−1) are unused sub-carriers called guard tones, and provided to prevent adjacent channels from being impeded. Here, the number of sub-carriers of the guard tones on both sides is two. However, guard tones the number of which is approximately two-tenth of the total number of sub-carriers are used in an actual system. For example, if there are 100 sub-carriers, the total of 20 sub-carriers are defined as being unused on both sides of the signal band.
In FIG. 2, d(2), d(N/2−1), d(N−3), etc. are known reference signals that are called pilot symbols and intended to detect synchronization. For the pilot symbols, a sub-carrier the frequency of which is the similar to the carrier wave frequency fc[Hz] is not used in normal cases. The reason is as follows: when being converted into a baseband signal, the carrier wave frequency fc becomes a DC component, on which 1/f noise is superimposed, an SNR therefore becomes small, and a signal error tends to occur despite the existence of a pilot symbol.
FIG. 3 illustrates the spectrum of the reception signal when a frequency characteristic exists in the receiver.
The example of FIG. 3 illustrates an influence exerted by 1/f noise existing in the vicinity of DC after the reception signal is frequency-converted into a baseband signal. The 1/F noise occurs in the quadrature demodulators 14-1, 14-2, etc. illustrated in FIG. 1. Additionally, a frequency characteristic, or the like, which degrades the signal power, can possibly exist in the vicinity of a particular frequency. In the case illustrated in FIG. 3, the symbols of d(N/2−1) and d(N/2+1) are degraded by the influence of the 1/f noise.
For data sub-carriers and the frequency selective fading of a transmission channel, there are effective measures such as a frequency interleaver, etc. With the frequency interleaver, not sub-carriers of a fixed frequency but sub-carriers of different frequencies are used when the data symbols of one sequence are respectively transmitted. However, for a synchronization detection method using a continuous pilot sub-carrier to which a particular frequency is allocated to transmit a pilot symbol, there are no effective measures although a high SNR is demanded for the pilot symbol. Also a scattered pilot sub-carrier, to which a temporally different frequency is allocated to transmit a pilot symbol, causes a similar problem if degradation is caused by a frequency characteristic in a range where the frequency of the pilot symbol changes.